A soft packaging container which is used as a pouch or a cup lid for packaging a food product, for packaging a medicinal product or for packaging cosmetics has been conventionally formed of a multi-layered film which is formed by laminating plastic films, a metal foil and the like. For example, JP 2003-118778 A, JP 2004-160860 A, JP 2004-243523 A and the like disclose a packaging container such as a pouch or a laminate tube formed of a multi-layered film.
In such a packaging container, in laminating different kinds of films such as a nylon film and a polyethylene film to each other, since both films have no adhesiveness, it is necessary to use an adhesive to make both films adhere to each other.
As an adhesive used in such a laminating operation, in general, a two-part curing type adhesive where curing starts only when a main agent and a curing agent react with each other is used. With respect to the two-part curing type adhesive, there has been known (i) a solvent adhesive where an adhesive resin is dissolved in a solvent and is applied by coating, (ii) a water-based adhesive where an adhesive is dissolved in water, (iii) a solventless adhesive which does not use a solvent and the like.
The solvent adhesive is an adhesive used in such a manner that a main agent and a curing agent are dissolved into solvents separately thus assuming a liquid state, and the main agent and the curing agent are mixed together immediately before being applied to a film by coating, and the mixture is applied to the film by coating. In general, a polyether-based main agent, a polyester-based main agent or an isocyanate-based curing agent is used.
However, the solvent adhesive evaporates a solvent when the solvent adhesive is applied to the film by coating and hence, it is not desirable in terms of an operation environment. Such a solvent adhesive is also not desirable in terms of the countermeasure to cope with the global environmental contamination. Accordingly, there has been a demand for the reduction of loads on an environment through the removal of VOC (Volatile Organic Compounds), the removal of CO2, the removal of toluene or the like from a viewpoint of conversion from a solvent adhesive to a solventless adhesive.
On the other hand, the solventless adhesive is a solvent used in such a manner that a main agent and a curing agent are formed into a liquid state by heating, are mixed together immediately before being applied to a film by coating, and the mixture is applied to the film by coating. As the solventless adhesive, in general, an inexpensive polyether-based resin is used.
However, when a polyether-based resin is used for forming a multilayered film for a packaging container, although the polyether-based resin exhibits favorable high-speed coating property, there exists a drawback that the polyether-based resin exhibits low laminate strength and low impact resistance. Under such circumstances, the polyether-based resin has been used for forming a packaging container for packaging junk food since there arises no problem even when the container exhibits low laminate strength and low impact resistance. However, a polyether-based resin is rarely used as a material of a packaging container for liquid contents which is required to exhibit favorable laminate strength and favorable impact resistance.
Further, as the solventless adhesive, besides a polyether-based adhesive, a polyester-based adhesive is also named. With respect to a conventional solventless polyester-based adhesive, when the adhesive is applied by coating at a coating speed which is a low speed of approximately 30 m/min, there arises no problem even when a manufactured packaging container is filled with a liquid content an disstored. However, when the adhesive is applied by coating at a high coating speed of 150 m/min, for example, there arise drawbacks such as defective appearance, insufficient laminate strength or insufficient impact resistance caused by insufficient wetting of the adhesive with a substrate.
For example, patent document 1 discloses a solventless two-part curing type adhesive composition which contains a polyol component (A) and a polyisocyanate component (B) and exhibits viscosity of 900 mPa·s or more at a temperature of 80° C. immediately after mixing the component (A) and the component (B). The polyol component (A) is polyol having a number-average molecular weight of 800 or more or a mixture thereof, while viscosity of the component (B) at a temperature of 25° C. is 20000 mPa·s or more.
Further, patent document 2 discloses a no-solvent adhesive composition which contains: 30 to 70% by weight of polyester polyol (A) having a number-average molecular weight of 500 to 2000; 20 to 50% by weight of polyester polyol (B) having a number-average molecular weight of 500 to 2000, 5 to 30% by weight of mono-functional alcohol (C) (the total of polyester polyol (A), polyester polyol (B) and mono function alcohol amounting to 100% by weight), a trifunctional isocyanate compound (D) formed of an alicyclic isocyanate compound or an aromatic diisocyanate compound, a trifunctional isocyanate compound (E) formed of an alicyclic isocyanate compound (here, a ratio between a total isocyanate group in the above-mentioned trifunctional isocyanate compounds (D), (E) and a total hydroxyl group in polyester polyol (A), (B) and mono-functional alcohol (C) (isocyanate group/hydroxyl group) being set to 0.5/1.0 to 1.5/1.0), and polybasic acid anhydride F.    Patent document 1: JP 2001-172602 A    Patent document 2: JP 2004-115681 A